Carbocations have long been regarded as important intermediates in organic reactions but are generally too unstable to be isolable.
Three types of carbocations have so far been isolated, and several of these have been structurally characterized by X-ray diffraction: (1) carbocations stabilized by heteroatoms in α-positions, notably O and N but including F and Cl; (2) carbocations with phenyl substituents; (3) tertiary-alkyl carbocations. These structural aspects have been summarized in Interactions between carbocations and anions in crystals. T. Laube, Chem. Rev. 1998, 98, 1277.
Carbocations carrying three alkyl substituents have proved isolable in several cases.
These include the parent ion CMe3+ as Sb2F11− salt (see: First X-ray crystallographic study of a benzyl cation, cumyl hexafluoroantimonate(V), and structural implications. T. Laube, G. A. Olah and R. Bau, J. Am. Chem. Soc. 1997, 119, 3087.) Low-temperature techniques are required to generate, crystallize and structurally characterize these compounds, which implies that their application is limited by their thermal instability and isolation techniques in SbF5-based media.
None of these examples contains Si or Sn substituents.
Cationic polymerization of olefins is known in the art.
Conventionally, cationic polymerization is effected using a catalyst system comprising: (i) a Lewis acid, (ii) a tertiary alkyl initiator molecule containing a halogen, ester, ether, acid or alcohol group, and, optionally, (iii) an electron donor molecule such as ethyl acetate. Such catalysts systems have been used for the so-called “living” and “non-living” carbocationic polymerization of olefins.
Catalyst systems based on halogens and/or alkyl-containing Lewis acids, such as boron trichloride and titanium tetrachloride, use various combinations of the above components and typically have similar process characteristics. For the so-called “living” polymerization systems, it is conventional for Lewis acid concentrations to exceed the concentration of initiator sites by 16 to 40 times in order to achieve 100 percent conversion in 30 minutes (based upon a degree of polymerization equal to 890) at −75° to −80° C.
Examples of the so-called “living” polymerization systems are taught in U.S. Pat. No. 4,929,683 and U.S. Pat. No. 4,910,321, the contents of each of which are incorporated herein by reference. Specifically, these patents teach the use of Lewis acids in combination with organic acids, organic esters or organic ethers to form cationic polymerization initiators that also create a complex counter anion. Apparently, the complex counter anion does not assist in or cause proton elimination.
In the so-called “non-living” polymerization systems, high molecular weight polyisobutylenes are prepared practically only at low temperatures (−60 to −100° C.) and at catalyst concentrations exceeding one catalyst molecule per initiator molecule. In practice, many of these catalyst systems are applicable only in certain narrow temperature regions and concentration profiles.
In recent years, a new class of catalyst systems utilising compatible non-coordinating anions in combination with cyclopentadienyl transition metal compounds (also referred to in the art as “metallocenes”) has been developed. See, for example, any one of:
published European patent application 0,277,003A;
published European patent application 0,277,004;
U.S. Pat. No. 5,198,401; and
published International patent application WO92/00333.
The use of ionising compounds not containing an active proton is also known. See, for example, any one of:
published European patent application 0,426,637A; and
published European patent application 0,573,403A.
U.S. Pat. No. 5,448,001 discloses a carbocationic process for the polymerization of isobutylene which utilizes a catalyst system comprising, for example, a metallocene catalyst and a borane.
WO-A 1-00/04061 discloses a cationic polymerization process which is conducted at subatmospheric pressure in the presence of a catalyst system such as Cp*TiMe3 (the “initiator”) and B(C6F5)3 (the “activator”). Such a system generates a “reactive cation” and a “non-coordinating anion” (NCA). Using such a catalyst system a polymer having desirable molecular weight properties may be produced in higher yields and at higher temperatures than by conventional means, thus lowering capital and operating costs of the plant producing the polymer.
The wide range of NCAs disclosed in WO-A1-00/04061 includes, boron, phosphorus and silicon compounds, including borates and bridged di-boron species.
The polymerization of isobutylene with small amounts of isoprene, to produce butyl rubber, presents unique challenges. Specifically, as is well known in the art, this polymerization reaction is highly exothermic and it is necessary to cool the reaction mixture to approximately −95° C. in large scale production facilities. This requirement has remained, notwithstanding advances in the art relating to the development of novel reactor designs and/or novel catalyst systems.
Further, it is the case that the copolymers so produced have markedly lower molecular weights than homopolymers prepared under similar conditions. This is because the presence of isoprene in the monomer feed results in chain termination by β-H elimination.
It would be desirable to be able to obtain high molecular weight isobutylene-based polymers, and in particular isobutylene-based copolymers, in high yield, at relatively high temperatures (as compared to the methods of the art) and under more environmentally-friendly conditions. This has not been demonstrated to date.